The present invention relates generally to a process and product for lubricating metal, particularly to such a process and product used prior to cold forming of the metal, and more particularly to lubrication of ferrous surfaces prior to metal deformation.
Chemical lubricant compositions span a broad range covering metal working fluids and coolants, of both oil and oil-in-water emulsions, and water based xe2x80x9csyntheticxe2x80x9d mixtures, as well as heavy oils, powders, pastes and greases, which all perform the lubricant functions of reducing contact friction, transferring heat, and allowing a physical or mechanical operation to take place.
xe2x80x9cCold formingxe2x80x9d of metal generally entails various work operations at room temperature such as extrusion of metal backward or forward; drawing; ironing; forming; and the like. For example, a metal wire may be drawn to a specific diameter through a die, or metal tubing to a specific wall thickness and diameter over a mandrel, or a metal shape may be formed through a press or heading machine. Each of these operations requires a continual reporting or lubricating layer having good xe2x80x9cslipxe2x80x9d characteristics, ie. no metal to metal contact, between the tooling and the work piece. The chemical and physical characteristics of this reporting layer determine the feasibility of the extrusion, as well as the tool life.
Presently, the majority of metal parts (such as steel parts) to be cold drawn are prepared with a zinc phosphate coating, and chemically reacted with a buffered sodium stearate lubricant; while in other situations, a zinc phosphate coating is coupled with a combined inorganic or organic lubricant referred to as non-reactive soap coating. For more severe extrusions, the zinc phosphate coated stock may be tumbled with molybdenum disulfide. This metal preparation generally includes the following steps: alkaline cleaning; a hot water rinse; a sulfuric acid pickle; a cold water rinse; a hot water rinse; a zinc phosphate application; a cold water rinse; an alkaline neutralizer; and an application of a reactive organic soap lubricant.
In the steel industry, unlike a layer of paint or oil on the steel surface, the zinc phosphate layer is a conversion coating, which reacts with the base steel and utilizes a portion of it into the coating itself. The resulting precipitation is a mixture of iron from the steel with zinc and phosphate from the phosphate solution which forms zinc-iron phosphate crystals that are chemically bonded to the steel surface. This crystal pattern builds uniformly across the steel surface during immersion in the zinc phosphate bath until a characteristic sparkling grey coating covers the surface. The phosphate coating does not inherently provide lubricity, but it does provide a non-metallic barrier between the steel being extruded and the tools performing the deformation. Although this barrier role is important, the greater function of the zinc phosphate is to retain and react with subsequent carriers and lubricants. These subsequent layers are important for providing relief at critical points of reduction and cold drawing when tools and dies need to allow the passing metal surface to flow freely and smoothly.
The steps involved in the coating deposition are pickling, acceleration, coating and sludge. These steps are repeated each time a clean steel piece enters the solution. After these steps, the steel surface has an anchored crystalline structure thereon which serves two purposes for lubrication. The first purpose is xe2x80x9cretentionxe2x80x9d of lime or stearate soaps, either applied in the pickle/process line or as a dry box lubricant prior to drawing. This is accomplished simply by the physical xe2x80x9cspongexe2x80x9d nature of the crystal lattice, which is able to hold large quantities of inorganic lime and/or organic soaps. For some subsequent forming operations, the sheer quantity of retained soap will be the basis for the success of the operation. In other instances, the composition of the retained soap will play the key role. In still other applications, both quantity and composition will be influential. The second purpose is the chemical xe2x80x9creactionxe2x80x9d with sodium and calcium stearate soaps. The stearate compounds, via a reactive soap tank or from within a dry box soap, displace the phosphate chemically connected to the zinc and bond directly to the zinc site. This bonded zinc stearate possesses a wide plastic range and high melting point necessary for difficult extrusions.
Current alternatives to the zinc phosphate procedure are oil replacements which are untidy, costly and environmentally unsound, as well as being of only limited effectiveness. In fact, the zinc phosphate procedure itself contains a high concentration of soluble zinc which requires profuse rinsing and subsequent waste water treating and hazardous disposal. Further, a lesser percentage of soluble nickel normally is incorporated into zinc phosphate mixtures, which faces even greater effluent restrictions. Still further, a natural and inevitable byproduct of the zinc process involves the generation of an insoluble, zinc bearing sludge in the process tank itself, which requires periodic solution decants and additional hazardous waste disposal. Yet another drawback lies in the fact that all of the solutions used in the zinc phosphate procedure have a relatively short lifespan in reference to square footage of metal processed and/or weeks of productivity. Desludging and discarding of solutions impede production, add costly labor time to the process, and contribute greatly to waste treatment costs and permanent hazardous disposal sites.
Several years ago, the first water based alternative to this chemical process was discovered and is described in detail in U.S. Pat. No. 5,484,541, which patent is incorporated herein by reference in its entirety. This process utilized water based, non-reactive lubricants and zinc phosphate-free chemical systems. As a result of information derived from utilizing this orthophosphate, acidic solution with either organic or inorganic acceleration (also described in the ""541 patent), an industry need became apparent. This need concerns the surface residue from the coating which remains behind after the forming, finishing and stress relieving operations are complete.
The ""541 invention deposited a heavy iron phosphate/iron oxide conversion coating on steel surfaces which provided a barrier between tooling and the metal being deformed. This barrier served two purposes: to act as an absorbent layer in order to increase the quantity of the subsequent non-reactive lubricant; and to provide a non-metallic layer in order to cushion the tooling from the part undergoing transformation, which prevents metal to metal contact, scratching and tool wear.
The benefits of this inter-metallic layer, although substantial, have been somewhat offset by its need for eventual removal. This remaining or partially removed, partially thermally degraded coating layer may in some cases cause surface irregularities, discoloration and/or cosmetic rejections, and may also become an eventual corrosion site, even under a rust preventative oil. Consequently, those using this coating system and not subjecting the formed pieces to certain minimal anneal time and temperature cycles, are required to alkaline clean the formed metal surface prior to any further finish processing. This removal is accomplished by means of a special heated alkaline soak immersion step and subsequent neutralized water rinse.
As such, a coating which would achieve the previously described two purposes (absorbency and cushioning), and yet not require removal would improve the efficiency and the economy of the product flow.
Thus, it is an object of the present invention to provide a process and product for lubricating metal prior to cold forming of the metal which is as, if not more effective than current zinc phosphate treatments, while being more efficient and more environmentally sound. Further, it is an object of the present invention to provide such a process and product which does not require profuse rinsing and subsequent waste water treating and hazardous disposal. Still further, it is an object of the present invention to provide such a process and product which does not require periodic solution decants and additional hazardous waste disposal of sludge. Yet still further, it is an object of the present invention to provide such a process and product which has a relatively long lifespan in reference to square footage of metal processed and/or weeks of productivity. Further, it is an object of the present invention to provide such a process and product which would not require cleaning of the formed metal surface prior to any further finish processing; and in fact advantageously provides corrosion resistance to the metal surface when left thereon.
The present invention addresses and solves the problems enumerated above. The present invention comprises an organic coating/deposit of a substantially powdery texture, loosely adhered to the metal/steel substrate by virtue of a slight metal etch and reaction with an organic, pH neutral solution. This deposit may be applied simultaneously with a lubricant, or alternatively in a pre-lubricant step. This deposit provides an absorbent cushion for lubricant deposition (whether applied prior to, or simultaneously with the lubricant) and metal deformation. The organic coating/deposit is an aqueous solution consisting essentially of a glucose derivative present in an amount sufficient to form a loose, soft and pliable layer of powdery crystals. The glucose derivative forms the greatest constituent within the coating. A meta-phosphate compound is present in an amount sufficient to provide linkage from the organic layer to the metal surface. The coating/deposit further consists essentially of a source of nitrate/nitrite in the form of metallic or alkali earth compounds, in an amount sufficient to provide limited acceleration for the organic deposit. The coating/deposit further optionally consists essentially of an anionic surfactant in an amount sufficient to provide wetting and drainage characteristics to the process solution.
A preferred composition for lubricating a metal workpiece prior to cold forming consists essentially of at least one aqueous alkaline stearate compound adapted to, and present in an amount sufficient to impart metal lubrication between about 180xc2x0 F. and about 320xc2x0 F.; at least one aqueous alkaline palmitate compound adapted to, and present in an amount sufficient to impart metal lubrication below about 160xc2x0 F.; at least one aqueous metallic stearate compound adapted to, and present in an amount sufficient to impart metal lubrication between about 240xc2x0 F. and 320xc2x0 F.; at least one polymeric glycol adapted to, and present in an amount sufficient to provide temperature stability, act as a carrier for the aqueous compounds, and impart a waxy lubrication to the metal; at least one hydrotropic agent adapted to, and present in an amount sufficient to solubilize the lubricating composition; and at least one stabilizing agent adapted to, and present in an amount sufficient to maintain solution stability among soluble constituents and to provide dispersing and suspending action among insoluble constituents. The lubricating composition further consists essentially of a dispersing agent for maintaining the aqueous compounds in solution; and a polyelectrolyte for imparting a cohesive film strength to the lubricating composition as it is dried on the metal workpiece.
The present invention further comprises a process for preparing/chemically coating/layering a metal workpiece outer surface prior to or simultaneous with lubrication and prior to subsequent cold forming, the process comprising the step of disposing an organic coating/deposit of a substantially powdery texture on the workpiece outer surface, the outer surface having been alkaline cleaned, hot water rinsed, acid pickled, and cold water rinsed. The first step of alkaline cleaning may sometimes be omitted if the incoming metal/steel is oil free.